Cold and Hot Forging: Fundamentals and Applications / Edited by Taylan Altan, Gracious Ngaile, Gangshu Shen
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234 / Cold and Hot Forging: Fundamentals and Applications
nology is still undergoing development, particu-
larly in aspects of surface treatment, lubrication,
and tooling.
Tooling for warm forging is similar to that for
cold forging, with some modifications made in
the die to allow increased temperatures, internal
die cooling, and venting of coolants.
REFERENCES
[Altan, 1970]: Altan, T., “The Use of Model
Materials in Predicting Forming Loads in
Metal Working,” Transactions ASME, May
1970, p 444.
[Altan et al., 1972]: Altan, T., and Beckner,
J.R., “Prediction of Punch Loads and Pres-
sures in Cold Extrusion of Steel,” SME Tech-
nical Paper MF72-142, Society of Manufac-
turing Engineers, 1972.
[Altan et al., 1982]: Raghupathi, P.S., Oh, S.I.,
Altan, T., “Methods of Load Estimation in
Flashless Forging Processes,” Topical Report
No. 10, Battelle Columbus Laboratories,
1982, p 94.
[Altan et al., 1983]: Altan, T., Oh, S.-I., Gegel,
H.L., “Metal Forming Fundamentals and Ap-
plications,” American Society for Metals,
1983.
[Altan et al., 1987]: Sevenler, K., Raghupathi,
P.S., Altan, T., “Forming Sequence Design for
Multistage Cold Forging,” Journal of Me-
chanical Working Technology, Vol. 14, 1987,
p 121–135.
[Altan et al., 1992]: Kim, H., Sevenler, K., Al-
tan, T., “Computer Aided Part and Processing
Sequence Design in Cold Forging,” J. of Ma-
terials Processing Technology, Vol 33 (No. 1–
2), 1992, p 57–76.
[Altan et al., 1996]: Altan, T., O’Connell, M.,
Painter, B., Maul, G., “Flashless Closed-Die
Upset Forging-Load Estimation for Optimal
Cold Header Selection,” Journal of Materials
Processing Technology, Vol 59, No. 1–2, May
1996, p 81–94.
[ASM, 1970]: Forging and Casting, Vol. 5, Met-
als Handbook, 8th ed., American Society for
Metals, 1970, p 72.
[Bay, 1994]: Bay, N., “The State of the Art in
Cold Forging Lubrication,” Journal of Mate-
rials Processing Technology, Vol 46, 1994, p
19.
[Bay, 1997]: Bay, N., “Cold forming of Alu-
minum—State of the Art,” Journal of Mate-
rials Processing Technology, Vol 71, 1997, p
76–90.
[Billigmann et al., 1973]: Billigmann, J., and
Feldmann, H.D., “Upsetting and Pressing,”
Carl Hansen Verlag, Munich, 1973 (in Ger-
man).
[Doehring, 1972]: Doehring, R.C., “New De-
velopments in Cold and Warm Forging,”
SME Technical Paper MF72-526, Society of
Manufacturing Engineers, 1972.
[Drozda, 1983]: Drozda, T.J., “Money Saving
Innovations in Automatic Forming,” Manu-
facturing Engineering, Vol 90, (No. 2), 1983,
p. 32–39.
[Feldmann, 1961]: Feldmann, H.D., “Cold
Forging of Steel,” Hutchinson and Company
Ltd., London, 1961.
[Feldmann, 1977]: Feldmann, H.D., “Cold Ex-
trusion of Steel,” Merkblatt 201, prepared for
Fig. 17.26
Effects of test temperature and test speed (strain
rate) on tensile strength and reduction of areas
of hot rolled type 1045 [Altan et al., 1983]
Fig. 17.25
Procedure for improving tool life [Yamanaka et
al., 2002]
Cold and Warm Forging / 235
Beratungsstelle fuer Stahlverwendung, Dus-
seldorf, 1977 (in German).
[Gentzsch, 1967]: Gentzsch, G., “Cold Upset-
ting, Cold Extrusion and Coining,” Vol. 1, Lit-
erature Review; Vol. 2, References, VDI Ver-
lag, Dusseldorf, 1967 (in German).
[Groenbaek, 1997]: Groenbaek, J., and Niel-
sen, E.B., “Stripwound Containers for Com-
bined Radial and Axial Prestressing,” Journal
of Material Processing Technology, Vo l 7 1
(No. 1), 1997, p 30.
[Herbst, 1967]: Herbst, V., “Accurate Shearing
of Workpiece Materials for Cold and Warm
Forging,” Doctoral dissertation, Technical
University, Hanover, Germany, 1967 (in Ger-
man).
[ICFG, 1992]: “Objectives, History and Pub-
lished Documents,” ISBN 3-87525-058-3, In-
ternational Cold Forging Group, 1992.
[ICFG, 2001a]: “Steels for Cold Forging:
Their Behavior and Selection,” Document
No. 11/01, International Cold Forging
Group, 2001.
[ICFG, 2001b]: “Warm Forging of Steels,”
Document No. 12/01, International Cold
Forging Group, 2001.
[ICFG, 2002]: “Cold Forging of Aluminum,”
Document No. 13/02, International Cold
Forging Group, 2001.
[James and Kottcamp, 1965]: James, C.T., and
Kottcamp, E.H., “Selection of Steel and Heat
Treatment for Ease of Cold Extrusion,” paper
presented at Chicago Technical Meeting of
the American Meeting of the American Iron
and Steel Institute, Oct. 20, 1965.
[Lange et al., 1985]: Lange, K., et al., “Hand-
book of Metal Forming,” McGraw-Hill, 1985,
p 2.3, 9.19.
[P.E.R.A., 1973]: “Cold Extrusion of Carbon
Steels,” P.E.R.A. Report No. 69, 1973.
[Pugh et al., 1966]: Pugh, H., et al., “Some As-
pects of Cold Extrusion of Steel,” Sheet Metal
Ind., Vol 43, 1966, p 268–305.
[Sagemuller, 1968]: Sagemuller, F., “Cold Im-
pact Extrusion of Large Formed Parts,” Wire,
No. 95, June 1968, p 2.
[Sahar, 1967]: “Prediction of Extrusion Pres-
sures in the Cold Forging of Steel,” Trans.
Metall. Society AIME, Vol 239, 1967, p 1461–
1468.
[Sevenler et al., 1987]: Sevenler, K., Raghu-
pathi, P.S., Altan, T., “Forming Sequence De-
sign for Multi-Stage Cold Forging,” J. of Me-
chanical Working Technology, Vol. 14, 1987,
p 121–135.
[Siebel, 1950]: Siebel, E., “Fundamentals and
Concepts of Forming,” Werkstattstechnik and
Maschienbau, Vol 40, 1950, p 373 (in Ger-
man).
[Watkins, 1973]: “Cold Forging and Extrusion
of Steel,” Review 176, International Metal-
lurgical Review, Vol 18, 1973 (Part I: Basic
Principles, p 123; Part II: Properties and Tool-
ing, p 147; Part III: Economics and Future
Applications, p 162).
[Wick, 1961]: Wick, C.H., ”Chipless Machin-
ing,” Industrial Press, New York, 1961.
[Yamanaka et al., 2002]: Yamanaka, M., and
Sunami, F., “Tool Design for Precision Forg-
ing,” presented at Cold and Warm Precision
Forging Workshop (Canton, MI), Yamanaka
Engineering Co., Ltd., Nov 14, 2002.
SELECTED REFERENCES
●
[Wick, 1984]: Wick, C.H., “Tools and
Manufacturing Engineers Handbook: Vol-
ume I, Forming,” Society of Manufacturing
Engineers, 1984.
●
[Witte, 1967]: “Investigation on the Varia-
tions of Loads and Energies in Cold Extru-
sion under Production Conditions, Influence
of Phosphate Coating Thickness and Lubri-
cants,” Report No. 6, Institute for Forming,
Technical University, Stuttgart, 1967, Verlag
Girardet, Essen. (in German).
CHAPTER 18
Process Modeling in Cold
Forging Using Finite-Element Analysis
Prashant Mangukia
18.1 Introduction
The finite-element method (FEM) is one of
the numerical techniques used for solving dif-
ferential equations governing engineering prob-
lems. This method has been applied to various
engineering processes, including metal forming.
The effectiveness of this method is now widely
recognized.
Finite-element methods for simulations of
metal forming processes are classified into those
for the elastic-plastic and rigid-plastic analyses.
In the elastic-plastic simulation, material is mod-
eled as deforming elastic-plastically. Thus, the
results are given for not only the plastic defor-
mation but also for elastic deformation such as
residual stresses and springback. Rigid-plastic
simulation assumes material to deform only
plastically, and, in comparison to elastic-plastic
simulations, it results in shorter computing time.
18.2 Process Modeling Input
As discussed in Chapter 16, “Process Mod-
eling in Impression-Die Forging Using Finite-
Element Analysis,” the accuracy of finite-ele-
ment (FE) process simulation depends heavily
on the accuracy of the input data, namely, flow
stress as a function of temperature, strain, strain
rate, and microstructure, as well as friction char-
acteristics at the interface. Other inputs required
are the geometric parameters of the objects and
the process parameters. Several issues, such as
material properties, geometry representation,
computation time, and remeshing capability,
must be considered in cost-effective and reliable
application of numerical process modeling. The
inputs to FEM are discussed in detail in section
16.3, “Process Modeling Input,” in Chapter 16
of this book. A brief summary is presented be-
low in terms of important inputs for modeling
the cold forging process.
18.2.1 Geometric Parameters
Depending on its geometrical complexity, a
forging process can be simulated either as a two-
dimensional, axisymmetric or plane-strain, or a
three-dimensional problem. In general, in order
to have an efficient simulation it is necessary to
remove all minor geometrical features, such as
small radii in the dies that do not have a signifi-
cant effect on the metal flow [Altan et al., 1990].
However, for some specific applications, such as
microforming processes, the size effects should
be taken into account in the simulation [Messner
et al., 1994].
18.2.2 Tool and
Workpiece Material Properties
In most cold forming simulations, tools are
considered as rigid. Temperature increases in
cold forging have little influence on the process.
Therefore, the simulations are usually carried
out under isothermal conditions. The stress-
strain relation of materials, commonly referred
to as the flow stress curve, is usually obtained
by the cylinder compression test. To be appli-
cable without errors or corrections, the cylindri-
Cold and Hot Forging Fundamentals and Applications
Taylan Altan, Gracious Ngaile, Gangshu Shen, editors, p237-246
DOI:10.1361/chff2005p237
Copyright © 2005 ASM International®
All rights reserved.
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238 / Cold and Hot Forging: Fundamentals and Applications
Fig. 18.1 Correction of flow stress data obtained from a compression test [Altan et al., 2001]
cal sample must be upset without any barreling
(Fig. 18.1), i.e., a state of uniform stress must
be maintained in the sample at all times during
the test. Use of adequate lubrication prevents
barreling to some extent. However, in practice,
barreling is inevitable, and procedures for cor-
recting flow stress errors due to barreling may
be necessary (see Chapter 4, “Flow Stress and
Forgeability,” in this book).
Figure 18.1(a) shows a sample that was upset
to 62% reduction (true strain of 0.96). This sam-
ple shows significant barreling, implying that the
lubricant applied was ineffective at this high
level of strain. The failure of the lubricant is evi-
dent by the two different regions (shiny and non-
shiny areas) depicted on the surface of the sam-
ple (Fig. 18.1a). In general, for process
modeling of precision forging, the experimental
flow stress curves were found to be reliable only
up to a strain level of 0.5 [Altan et al., 2001].
One way of correcting the flow stress, or de-
termining approximately the error magnitude at
high strain levels caused by inadequate lubri-
cation (barreling), is to simulate the compression
process by FEM using various friction factors
(Fig. 18.1c) [Altan et al., 2001]. By measuring
the amount of barreling (Fig. 18.1a) and com-
paring the experimental data with FEM predic-
tions, it is possible to obtain more reliable flow
stress data at higher strain levels. This method,
called the inverse analysis technique, has been
used to simultaneously determine friction and
stress/strain data using a ring test [Altan et al.,
2003].
Attention should also be given to other
sources of flow stress error determined through
compression tests, such as variation in coil ma-
terial properties, specimen preparation, surface
defects, parallelism of platens, and specimen
size.
Though in most FE simulations the tools are
considered rigid, this assumption may not hold
for complicated forgings with tight tolerances,
particularly in microforming applications.
Hence, depending on the part complexity, elas-
tic-plastic properties of the tools/dies become an
important component in the simulations. The de-
tails of flow stress determination and inverse
analysis can be found in Chapter 8, “Inverse
Analysis of Simultaneous Determination of
Flow Stress and Friction,” of this book.
18.2.3 Interface Conditions
(Friction and Heat Transfer)
The friction and heat-transfer coefficient are
not readily available in literature. The most com-
mon way to determine the shear friction factor
in forging is to perform ring compression tests.
Friction factors measured with the ring com-
pression test, however, are not valid for preci-
sion cold forging processes (hot, warm, and
cold) where the interface pressure is very high
and the surface generation is large. The friction
conditions change during the process due to
changes in the lubricant and the temperature at
the die/workpiece interface. In such applica-
tions, the double-cup extrusion test is recom-
mended for estimation of the friction factor (see
Chapter 7, “Friction and Lubrication,” in this
book).
Process Modeling in Cold Forging Using Finite-Element Analysis / 239
Fig. 18.2 Automotive axle shaft with chevrons [Hannan et al., 2000]
18.3 Process Modeling Output
The process modeling provides extensive in-
formation on the forging process. The output of
process modeling can be discussed in terms of
the metal flow, the distribution and history of
state variables, the equipment response during
forging, and the microstructure of the forging,
as discussed in Chapter 16, “Process Modeling
in Impression-Die Forging Using Finite-Ele-
ment Analysis,” in this book.
18.4 Process Modeling Examples
A few case studies from the literature are dis-
cussed below to examine the applications of
FEM in cold forging processes.
18.4.1 Prediction and
Elimination of Defects in Cold
Extrusion [Hannan et al., 2000]
In the automotive industry, many shaft and
shaftlike components, including fasteners, are
produced by forward extrusion. Some of these
components are critical for vehicle safety and
must be free of defects. These defects could be
visible external ones, such as laps or cracks, or
nonvisible internal defects, such as chevron
cracking (Fig. 18.2) [Hannan et al., 2000].
Spherical dies are sometimes used in extru-
sion of safety parts, because they provide more
uniform grain flow around corners and improved
dimensional controls on net shape surfaces.
Therefore, it is beneficial to investigate the oc-
currences of chevron cracks in extrusion with
spherical dies.
The objectives of this study were to measure
the critical damage values for typical materials
used in cold extrusion and to develop criteria or
guidelines for designing forward extrusion dies
for producing chevron-free extrusions. Ductile
fracture can be defined as a fracture that occurs
after a component experiences a significant
amount of plastic deformation and is influenced
by numerous parameters, including the defor-
mation history of the workpiece material and the
process conditions (i.e., rate of deformation,
lubrication, and friction). Other factors that in-
fluence fracture include chemical composition,
microstructure, surface conditions, and homo-
geneity.
At the Engineering Research Center for Net
Shape Manufacturing (ERC/NSM), a method-
ology for predicting ductile fracture via FE sim-
ulations was developed, as shown in Fig. 18.3.
It was based on the modified Cockroft and La-
tham criterion. Assuming the critical damage
value is a material constant, several tests should
be performed to obtain, for a given material, the
flow stress and the critical damage value (CDV).
Once the CDV for a specific material is ob-
tained, it is possible to predict through FEM
simulations the formation of cracks in forming
operations. Two different materials were se-
lected, details of which could be found in [Han-
nan et al., 2000]. Figure 18.4 shows that FEM
can successfully predict the formation of chev-
ron cracks in forward extrusion using this meth-
odology. The commercial FE code DEFORM
(Scientific Forming Technologies Corp.) was
used for this study.
In summary, the CDV concept can be used to
predict the possibility of fracture formation in
cold extrusion. Thus, using this concept and
FEM simulation, die design and process se-
quence can be modified to avoid fracture defects
in cold extrusion.
18.4.2 Modeling of Golf Ball Mold
Cavities [Hannan et el., 1999]
Golf ball mold cavities are manufactured by
the hobbing process. This process allows the
240 / Cold and Hot Forging: Fundamentals and Applications
Fig. 18.4 Simulation of chevron cracks and experimental validation [Hannan et al., 2000]
Fig. 18.3 Methodology to predict and prevent the formation of cracks in metal forming operations [Hannan et al., 2000]
molds to be manufactured more efficiently and
reduces the cost. Finite-element modeling was
used in this study to design the tooling. A para-
metric analysis was conducted to evaluate the
effect of various process conditions on the ma-
terial flow and forming load for the hobbing pro-
cess. For the parametric study, the tool design
shown in Fig. 18.5 was used.
The results of FEM simulations showed that
a design can be formulated that resulted in a
forming load that was realistic and did not sur-
pass the yield strength of the tool material. How-
ever, parametric analysis also showed problems
with material flow of the preform. In each simu-
lation, dimples did not fill completely, as shown
in Fig. 18.6.
Using the information from this study, a de-
sign that resulted in good material flow (all
dimples filled) and low forming loads was de-
veloped. Two-dimensional (2-D) and three-di-
Process Modeling in Cold Forging Using Finite-Element Analysis / 241
Fig. 18.6 Incomplete filling of the dimples, as predicted by two-dimensional FEM simulations [Hannan et al., 1999]
Fig. 18.5 Tool setup and studied parameters in FEM [Hannan et al., 1999]
mensional (3-D) FEM simulations were used to
analyze the modified design that fills the dimples
of the lower punch completely and at realistic
loads. The details of the study can be found in
[Hannan et al., 1999].
18.4.3 Design of Automotive Parts
Net shape forging of power steering pinions
with helical teeth, helical gears, and inner races
presents a great challenge. However, the use of
commercial 3-D FE software in process devel-
opment has drastically reduced the production
costs by eliminating multiple trials and, at the
same time, has improved part quality, tool life,
and geometric complexity of forgings. Figure
18.7 shows the application of FE simulations in
the design and development of the helical extru-
sion process for manufacturing the pinion
shown. The advantages of using FEM to simu-
late and hence design the optimum process were
[Yamanaka et al., 2002]:
●
Obtain a better drive feeling
●
Generate the tooth profile freely
●
Establish an iterative technique for die and
process design as well as heat treatment
Figure 18.8 shows the simulation of cold forg-
ing of an automotive part. The actual part is
shown, which has underfill and fracture. The
one-die two-blow process used to manufacture
this part was successfully simulated, and the re-
sults show similar underfill and explain the frac-
ture in the part.
18.4.4 Microforming of Surgical Blades
The trend in miniaturization allows the pro-
duction of cold forged parts with dimensions
242 / Cold and Hot Forging: Fundamentals and Applications
Fig. 18.8 Cold forging of an automotive part (a, b, c, and d are the stages of forming). Courtesy of DEFORM
Fig. 18.7 Development of steering pinion with the aid of FE simulation [Yamanaka et al., 2002]
less than 0.04 in. (1 mm) range for electronics
and biomedical applications. These parts are
currently produced by 3-D etching and other
metal-removal processes. Microforming is a po-
tential process for mass production of net shape/
near-net shape microcomponents. However, for
microforming to be cost-effective and competi-
tive, a comprehensive knowledge pertaining to
the following factors is needed: scale effects/mi-
croplasticity, effect of microstructure on the pro-
cess, relative stiffness of the tooling, and process
control and capability.
With the aid of FEM, the interrelationships
between these variables can be studied so as to
Process Modeling in Cold Forging Using Finite-Element Analysis / 243
Fig. 18.9
Microforming of surgical blades. Blank thickness ⳱ 0.004 in. (0.1 mm); final blade thickness ⳱ 0.0004 in. (0.01 mm)
[Palaniswamy et al., 2001]
Fig. 18.10 Double-cup extrusion test for determination of friction conditions in microforming [Tiesler et al., 1999]
Fig. 18.11
Possible areas of underfill in cold forging of a
cross-groove inner race [Vazquez et al., 1996]
provide guidelines for developing microforming
processes. In cooperation with industry, the
ERC/NSM has developed a microforming pro-
cess for making surgical blades. Figure 18.9
shows an example of 3-D FE simulations for a
surgical blade with initial blank thickness of
0.004 in. (0.1 mm) and final edge thickness of
0.0004 in. (0.01 mm) [Palaniswamy et al.,
2001].
Due to the high surface-to-volume ratio in the
microforming process, friction becomes even
more important than in conventional forging. Ef-
fects of miniaturization on friction have been in-
vestigated by using the double-cup backward
extrusion test, with oil as a lubricant (Fig.
18.10). Comparisons between experiment and
simulation show that friction effects increase
with a decrease in size of the specimen. The fric-
tion observed for a 0.04 in. (1 mm) diameter
billet was four times that for a 0.16 in. (4 mm)
diameter billet.
18.4.5 FEM Analysis of Process Design to
Cold Forge a Cross-Groove
Inner Race [Vazquez et al., 1996]
Figure 18.11 shows the cross-groove inner
race. Cold forging of this part is very difficult,
244 / Cold and Hot Forging: Fundamentals and Applications
Fig. 18.12 Example of FEM analysis of forging process sequence design [Kim et al., 1996]
as the grooves of the inner race act as undercuts.
Thus, the objective of this study was to develop
a new design concept for tooling to improve the
tolerance that is achievable in the cold forged
groove. Figure 18.11 shows the possible areas
(marked as B) of underfill. Two plausible de-
signs were chosen for investigation from several
other designs. Physical modeling and FE anal-
ysis were used to analyze the designs selected.
Two-dimensional FEM simulations were per-
formed for preliminary investigation of the
metal flow. DEFORM was used for both 2-D
and 3-D FEM simulations. The results of 2-D
simulations give a window of operation for fur-
ther investigation, though they may not accu-
rately predict all the parameters. In order to ob-
tain a more accurate prediction of the metal flow,
strains, and the contact pressure distribution be-
tween the billet and the die, 3-D FEM simula-
tions (one-sixth of the billet, due to symmetry)
were conducted. The calculations showed that,
at the end of the stroke, the die cavity pressure
increases considerably. Since the maximum
pressure should not exceed a certain limit, it is
necessary to control the motion and pressures of
the inner punches. This study clearly illustrated
that in the multiaction forming process of the
cross-groove inner race, a compromise between
cavity filling and pressure must be achieved.
Three-dimensional FEM simulations of different
concepts for the punch motion help to select the
best tool design before the actual tooling is built.
The details of this study are given in [Vazquez
et al., 1996].
18.4.6 Computer-Aided Design
System and FEM Application for
Process Design in
Cold Forging [Kim et al., 1996]
Instead of trying to develop computer systems
to automate the forming sequence design, the
goal should be to develop computer-aided tools
to assist die designers, while they “do” the de-
sign. Such a computer-aided design (CAD)
system was developed at ERC/NSM. It consists
of three parts: a computer program called
FORMEX, a commercial CAD program, and a
commercial FE program, DEFORM. In order to
increase the capabilities of FORMEX, forging
sequences collected from references and indus-
try were entered in the “sequence library.” Fi-
nite-element simulations were conducted to
evaluate the sequence generated. Figure 18.12
shows an example of FE simulation of such a
sequence. This shows that FE simulations of